Hybrid electric vehicles or HEV can selectively utilize different energy sources as needed in order to achieve optimal fuel economy. A typical HEV can use an internal combustion engine as a primary vehicle propulsion mechanism, and a high-voltage battery module or energy storage system (ESS) for restarting the engine and/or for electrical propulsion of the HEV. For example, an HEV having a typical full hybrid powertrain can be propelled via purely electrical means up to a threshold speed, with one or more motor/generators drawing power from the ESS as needed. Above the threshold speed, the engine can provide some or all of the required propulsive torque. By way of contrast, an HEV having a limited hybrid powertrain lacks means for electrical propulsion, but retains certain functionality such as regenerative braking capability and the selectively powering off or shutting down of the engine at idle; fuel-saving performance capabilities that are generally shared with the full hybrid design.
The ability of an HEV to automatically shutdown the engine is a capability referred to hereinafter as “Auto Stop” functionality, which allows otherwise wasted fuel to be conserved during certain vehicle conditions. In vehicles having Auto Stop functionality, a motor/generator also referred to as a belt alternator starter (BAS) is used in lieu of a conventional alternator to apply torque to a serpentine belt of the engine whenever a driver signals intent to resume travel from an Auto Stop state. Torque from the BAS can turn the engine for a transient duration until a flow of fuel from the vehicle fuel supply can be restored.
During an Auto Stop state, components normally driven by the engine no longer operate. The main hydraulic pump for transmissions requiring pressurized oil to maintain clutch pressure and air conditioning compressor pumps driven by the engine's serpentine belt are two examples. During an Auto Stop event, a transmission auxiliary pump can be automatically energized in order to provide oil pressure to the transmission that would normally be supplied by the engine-driven transmission pump. Additionally, if the air conditioning (A/C) compressor is driven via the engine's serpentine belt, the A/C compressor is by necessity disabled during the Auto Stop event.
However, under some circumstances it may not be desirable to disable HVAC devices, such as during an active defrost mode, when the ambient temperature and/or humidity is relatively high, and/or when the driver of the HEV manually overrides Auto Stop functionality. The requirement to maintain engine operation for HVAC or other vehicle performance issues can complicate the diagnosing of the transmission auxiliary oil pump since the engine-driven oil pump is operational. Enabling the auxiliary oil pump while the engine is rotating combines the output of both pumps, and does not offer a clear indication of the auxiliary pump's output alone. Conventional methods of diagnosing an auxiliary pump include forcing an Auto Stop event to execute, allowing Auto Stop during all A/C modes, and/or using a pressure switch in conjunction with the HEV ignition to automatically test the auxiliary pump whenever the ignition switch is turned off. Each of these alternatives can be less than optimal, as they each involve either or both of shutting down the engine and the use of additional hardware.